Terminology question: does one define an XML vocabulary, an XML grammar,
an XML language (".... Language language"), or something else?

I think that an XML schema is something that describes "the thing", not
the thing itself (feel free to argue otherwise).

I tend to use the term "XML vocabulary", because I think that I am
defining something that follows the grammar of XML. But I can see how one
might say that an "XML grammar" (as Shomi uses the term) introduces its
own specific grammatical rules (that is, not just names), within the
bounds of XML grammar, and is therefore itself a grammar.